Potentiometer drive unit



March 4, 1969 o; H. SAHMEL POTENTIOMETER DRIVE UNIT I of 2 Sheet Filed Aug. 16, 1967 OTTO H. SAHMEL INVENTOR.

ATTORNEY O. H. SAHMEL POTENTIOMETER DRIVE UNIT March 4, 1969 Sheet 3 of2 Filed Aug. 16, 196? orro H. SAHMEL INVENTOR. BY [14% ATTORNEY United States Patent 3,431,531 POTENTIOMETER DRIVE UNIT Otto H. Sahmel, Rochester, N.Y., assignor to Bausch & Lomb Incorporated, Rochester, N.Y., a corporation of New York Filed Aug. 16, 1967, Ser. No. 661,015 US. Cl. 338-196 11 Claims Int. Cl. H01c /00 ABSTRACT OF THE DISCLOSURE A servo driven indicator is movably mounted on an elongated shaft having a helical drive track. A potentiometer is mounted on the drive shaft comprising at least one elongated resistance element disposed along the helical track. A contact on the indicator provides a sliding connection with the resistive element.

Background of the invention This invention relates to drive units for servo controlled positioning systems.

In the usual recorder or other types of similar servo controlled positioning instruments, the indicator or movable element is mounted to move back and forth along an elongated shaft having a screw type drive. The shaft is rotated by a servo motor in a closed loop feedback system. The positional feedback signals are generally provided by a multiturn potentiometer gear coupled to shaft. The gearing to the potentiometer is generally considered a source of error, such as backlash and friction, reducing the accuracy of the system.

Summary of the invention An elongated shaft including a helical track provides a reversible drive for positioning a guide member, such as a controlled element, in a servo feedback positioning system. A potentiometer is mounted on the shaft having at least one elongated resistive element disposed along the helical track and including slip type connections at the ends for energizing the resistive element. A contact element is included as a portion of the indicator device for engaging the resistive element to provide a variable pickup. The potentiometer and shaft function as an integral unit eliminating positional backlash error and reducing frictional error.

A further feature of the invention includes two elongated resistive elements separately positioned along the track, such as on opposite sides of the helical track, wherein one element is linear and the other non-linear. The non-linear resistive element may, for example, be formed with a logarithmic or exponential function. Two contacts are provided on the indicator device for separately engaging the individual resistive elements.

Brief description of the drawings FIGURE 1 is a perspective view of the potentiometer drive unit of the invention.

FIGURE 2 is a top view of the guide member of FIGURE 1.

FIGURE 3 is a sectional view of the guide member of FIGURE 2 taken along the lines 33.

FIGURE 4 is a rear view of the guide member of FIGURE 2.

FIGURE 5 is a front view of the guide member with a portion cut away to illustrate the potentiometer contacts included within the guide member.

FIGURE 6 is a side view of the guide member without the helical drive and having a recorder pen attached thereto.

3,431,531 Patented Mar. 4, 1969 Description of the preferred embodiments The potentiometer drive unit of FIGURE 1 includes an elongated drive shaft 10 formed with a helical track 11 and two cylindrically shaped ends 12 and 13. The shaft ends 12 and 13 are mounted for rotating motion in the bearings 14 and 15. The shaft and helical track combination is illustrated as a commercially available bar screw having an elongated bar with a rectangualr shaped cross section (FIGURE 3) twisted to form the helical or spiral track. A servo motor 16 is coupled through the pulleys 18, 20, and the belt 22 to reversibly rotate the spiral shaft 10 when energized.

The drive shaft 10 includes two elongated resistive elements 24 and 26 disposed on opposite sides of the helical track 11 and insulated therefrom. The two resistive elements can for example comprise an elongated flexible core with a resistive wire continuously wound around its circumference or a resistive compound suitably deposited on the shaft. One of the resistive elements can for example exhibit a linear characteristic versus length, while the other a non-linear characteristic, such as logarithmic. The opposite ends of the resistive elements 24 and 26 are suitably connected to the slip rings 2831. Slip ring contacts 32-35 provide for the selective energization of the resistive elements,

An indicator, such as a recorder pen 40, is mounted upon a guide member 42 for reversible motion in response to the rotation of the drive shaft 10. A second guide member 44 is coupled to the guide member 42 to provide a resilient force to reduce backlash or deadband in the motion of the guide member 40 in response to the rotation of the drive shaft 10. Both of the guide members 42 and 44 include an opening 46 (FIGURE 6) at each end having two rectangular shaped notches 48 that engage the track 11. The described bar screw and guide member combination are well known as linear motion bar screws and require no further explanation.

The guide member 42 includes a pair of openings 50 and 52 (FIGURE 3) formed on opposite sides (top and bottom) to provide access to a pair of contact mounting screws 54 and 56. The screws 54 and 56 fasten the potentiometer contacts 58 and 60 so that they resiliently urge against the resistive elements 24 and 26. The guide member 40 can be made of non-conducting material, or the contacts 58 and 60 can be suitably insulated from the guide member. A common connection 64 is provided between the contacts 58 and 60. The contacts 58 and 60 are disposed in the direction of the spiral track (FIGURE 5) to provide a sliding contact with the resistive elements 24 and 26. The contact 58 extends beyond the screw 58 (FIGURE 3) to make a slidable resilient contact with a conductive pickoff rod 62. The guide member 44 is coupled to the guide mem ber 42 by a screw 70 (FIGURE 2) that extends through an opening 72 in the guide member 44 and makes a threaded connection with the guide member 42. A spring 74 provides a resilient force between the guide members 42 and 44. The spring connection between the guide members resiliently urges the notches 48 in both the guide members against the helical track to provide an anti backlash function.

The pen 40 is attached to the guide member 42 by a resilient clip so that the pen can be readily removed for cleaning.

The rod 62 is pivotally mounted to the shaft ends 12 and 13 by a pair of insulated pivot arms 76 and 78. A flexible wire 80 is connected to one end of the rod 62 to provide an electrical contact thereto, A spring 82 is connected between the pivot arm 76 and a stationary base- 84 to provide a resilient force on the guide members 42 and 44 to pivot the pen 40 away from its recording position in a direction as designated by the arrow 86. A solonoid arm 88 is pivotally connected to the pivot arm 76 at a point opposite that of the spring 82. When the solenoid 90 is energized the pen 40 is pivoted into its recording position.

In operation, the resistive elements 24 and 26, the contacts 58 and 60 and the conductive rod 62 function as a potentiometer. The particular resistive element employed is controlled by its energization. For example, if a linear recording function is desired, the linear resistive element is energized by applying a potential to its associated slip ring contacts. Alternately, a non-linear function can be provided by energizing the slip ring contacts associated with the non-linear resistive element.

If it is desirable to employ both resistive elements 24 and 26 simultaneously, such as to record linearly and generate a non-linear (logarithmic) signal of the recorded signal, separate pickoffs can be provided for each resistive element as illustrated in FIGURE 7. The contacts 58' and 60' are insulated from each other (by removing the connection 64) and two conductive bars 92 and 94 are provided for the contacts 58 and 60 respectively. In this embodiment both the resistive elements 24 and 26 can be energized simultaneously and the potentials picked off individually.

The potentiometer drive unit of the invention has the advantage of providing a positional feedback signal with a minimum of error by eliminating any deadband due to gearing to the potentiometer and friction therefrom. Any backlash in the guide member 42 is minimized by the anti-backlash function of the guide member 44. In addition to the foregoing, by disposing the resistive elements along the spiral track, a substantially longer resistive element is achieved as compared to a linear resistive element positioned adjacent the shaft thereby allowing for greater accuracy.

Although the potentiometer drive unit has been described in the content of an indicator device or recorder drive, it is to be understood that the invention will also apply to any type of reversible drive unit employing a potentiometer positioned pickotf.

What is claimed is:

1. A combined indicator drive shaft and potentiometer for servo controlled measuring apparatus comprising:

an elongated helical screw unit;

an elongated resistance element disposed along the helical screw unit; slip connection means coupled to opposite ends of said resistance element for applying a potential thereto;

guide means mounted on said helical screw unit and engaging said helical screw unit to the exclusion of said resistance element for movement along said screw unit in response to rotation of the screw unit;

contact means mounted on said guide means for movement with said guide means and slidably contacting a portion of said resistance element;

means coupled to said contact means for transmitting an electrical signal from said guide means, and

an indicator mounted on said guide means for providing an indication the position of said guide means along said screw unit.

2. The combination as defined in claim 1 wherein:

said helical screw unit comprises a shaft having a substantially rectangular cross section and formed so that the edges of the rectangle comprise a spiral track, and

said guide means engages said spiral track to the exclusion of said resistance element for movement along said spiral track in response to the rotation of said shaft.

3. The combination as defined in claim 2:

4 wherein said resistive element is disposed along said helical tIack;

including a second resistive element disposed along said helical track but separated from said other resistive element, and

second contact means is mounted on said guide means for movement with said guide means and slidably contacting a portion of said second resistive element.

4. The combination as defined in claim 3 wherein:

one of said resistive elements exhibits a linear resistance characteristic and the other of said resistive elements exhibits a non-linear resistance characteristic.

5. A combined marker drive and potentiometer for a servo controlled recording instrument comprising:

marker drive means including a rotatably mounted elongated shaft having a helical track;

an elongated resistive element disposed along said helical track;

slip connection means adjacent opposite ends of said shaft coupled to the ends of said resistive element for applying a potential thereto;

guide means mounted on said shaft and engaging said helical track for movement along said shaft in response to a force from said track as said shaft is rotated;

a contact mounted on said guide means for movement with said guide means and engaging a portion of said resistive element;

a means for mounting a marker on said guide means;

means coupled to said contact for transmitting electrical signals;

a record medium is positioned for receiving said marker device, and

means for pivoting said guide means about said helical track for engaging said marker device to said record medium.

6. The combination as defined in claim 5 wherein:

a second elongated resistive element is disposed along the helical track separated from the other resistive element, and

a second contact is mounted on said guide means for movement with said guide means and engaging a portion of said second resistive element.

7. The combination as defined in claim 6 wherein:

one of said resistive elements presents a linear resistance characteristic, and

the other one of said resistive element presents a nonlinear resistance characteristic.

8. The combination as defined in claim 7 wherein:

said non-linear resistance element exhibits an exponential characteristic.

9. The combination as defined in claim 5 wherein said means coupled to said contact for transmitting electrical signals comprises a conductive rod and a sliding connection between said contact and said rod.

10. A combined indicator drive shaft and potentiometer for feedback servo measuring systems comprising:

an elongated rotatably mounted shaft including a helical track;

first and second elongated resistive elements disposed along the helical track but separated from each other;

slip connection means coupled to opposite ends of said resistive elements for applying electrical signals thereto;

a guide member mounted on said shaft for engaging said helical track to the exclusion of said first and second resistive elements for movement along said shaft in response to a force from said helical track as said shaft is rotated;

an indicator mounted on said guide member for indi cating the position of said guide member, and

contact means mounted on said guide member provid- 5 6 ing slidable contact with a portion of said resistive References Cited elements. UNITED STATES PATENTS 11. The combination as defined in claim 5 wherein: 1,535,898 4/1925 Camp 338 145 said means coupled to said contact for transmitting 2 432 327 12 1947 Moore 333 195 X electrical signals comprises a conductive rod and a 5 2,463,814 4/1949 V Al 338 89 X sliding connection between said contact and said rod, 2,972,124 2/ 1961 Blanco 33889 X and 3,333,223 7/1967 Mucher 338145 said means for pivoting said guide means includes a 2,938,396 5/1960 Nicolosi et a1 338-443 X solenoid coupled to said conductive rod so that in response to the actuation of said solenoid said rod 10 ROBERT SCHAEFER Primary Exammer' causes said guide means to pivot. H. J. HOHAUSER, Assistant Examiner. 

